Feeding by a Lotic Mayfly Grazer as Quantified by Gut Fluorescence

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Abstract
It is often of interest in ecological research to quantify the feeding of study animals. Approaches used previously with grazing aquatic insects have been either labor-intensive (algal cell counts of gut contents), lacking in sensitivity (AFDM, relative gut fullness), or involve radioactive tracers (dual isotope method). A method developed and applied by marine biologists to study zooplankton feeding rates uses the amount of plant pigment in the gut as a feeding index. We have modified this technique to assess feeding by larvae of a grazing mayfly, Baetis bicaudatus (Ephemeroptera: Baetidae) in a western Colorado [USA] stream. Gut pigments include chlorophyll from ingested algae, which is rapidly degraded to phaeophorbide and passes unassimilated through the gut, thereby acting as a tracer of feeding activity. Our experiments showed that the optimum dissection and pigment extraction procedure entailed macerating whole Baetis individuals and extracting the gut pigments for 24 hr in 90% ethanol. Pigment levels (chlorophyll a and phaeopigment a) were quantified on a Turner Model 112 fluorometer and expressed as ng total pigment per animal. The technique was reliable using individual mayflies as the sample unit. Storage of intact animals in extraction bottles with a small amount of solvent for 8 hr before processing did not result in degradation of pigment, which added flexibility to the procedure. We foresee applications of this approach in studies of relative feeding rates (daily, seasonal, phylogenetic patterns) and feeding under experimental conditions involving manipulation of factors such as predation, competition, food availability, and the physical environment.

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